This invention relates to methods of beneficiating wood, and more particularly to methods of effecting dimensional stability and other desired properties in wood through aldehyde cross-linking of cell wall structural constituents of the wood.
Many problems in the effective utilization of wood arise from physical shrinking and swelling due to variations in moisture content below the fiber saturation point of the wood. Seasonal variations in ambient humidity commonly lead to seasonal cycles of shrinking and swelling of wood and wood products. In a similar manner, the movement of wood products between regions of varying humidity commonly results in shrinking or swelling of the wood. This dimensional instability of wood can result in serious physical problems in wood products, such as cracking and/or warping of the wood, unacceptable size variations in doors, windows, and drawers, and the like, as well as surface problems such as surface checking, grain raising, and the like. In addition to the foregoing, wood is subject to fungal decay and insect damage, both of which effectively limit the expected useful life of wood products in various regions.
In the past, various wood preservatives have been employed to protect certain desirable properties of wood products. However, commonly used wood preservatives, such as creosote and pentachlorophenol, have associated environmental toxicity, flammability, odor, color and handling problems which severely limit their desirability in many applications. Wood products are also commonly protected against the effects of contact with surface moisture by applying a vapor barrier, such as paint, varnish or oil, on the surface of the wood. Although being relatively effective in protecting the wood surface against immediate weather degradation, it has been found that vapor barriers afford little or no protection against long-term effects, such as ambient humidity variations.
In order to overcome the foregoing problems, many attempts have been made to impart dimensional stability to wood and wood products, primarily through bulking, reducing the affinity of wood for moisture, or limiting the accessibility of wood to moisture by effecting cross-linking of cell wall structural constituents of the wood.
Bulking implies swelling of wood to its maximum dimension and retaining the wood at that dimension by depositing a bulking agent, such as polyethylene glycol, in the wood cell wall and in the cell cavity. Diffusion of the bulking agent from the cavity or lumen into the cell wall is typically a slow process, often requiring several days for adequate treatment. In addition, as much as 30% or more of a bulking agent by weight of dry wood, is frequently considered to be a minimum acceptable level for most practical applications. For these reasons, bulking is often an economically prohibitive means of attaining dimensional stabilization of wood. The resulting wood product also has been found to have relatively low strength properties on the order of those normally associated with green wood.
The affinity of wood for moisture can be reduced, and dimensional stability can be improved, by chemically modifying a substantial number of hydroxyl groups associated with the cell wall to substantially reduce hydrogen bonding with water. For example, acetylation of the hydroxyl groups can be obtained by treating wood with acetic anhydride in the gaseous state at elevated temperatures with either pyridine as a catalyst or xylene as a carrier solvent. It has been found, however, that the high cost of chemicals employed, and associated equipment corrosion problems, often makes acetylation an economically prohibitive dimensional stabilization alternative.
It is also known in the art that dimensional stabilization of wood may be obtained by using formaldehyde as a cross-linking agent to form methylene bridges between the hyroxyl groups of the wood carbohydrate polymers. Formaldehyde cross-linked wood has been shown to exhibit greatly improved dimensional stability properties over those associated with untreated wood. However, in prior art cross-linking processes, acid catalysts have been required to promote the cross-linking reaction. In general, it has been known in the art that the more acidic the catalyst, the more complete the cross-linking reaction, and that relatively weak acids, such as acetic acid, have been ineffective to promote polymerization. Due to the highly acidic environment required in prior art aldehyde cross-linking processes, gains in dimensional stability have been made at the expense of other highly desirable physical properties of the wood, such as toughness, abrasion resistance, and other strength properties. For example, wood in which shrinking and swelling has been reduced by 60% through conventional, acid catalyzed formaldehyde cross-linking has been found to retain only 20-30% of its original toughness and 7% of its original abrasion resistance. The degradation of desirable wood properties through conventional acid catalyzed aldehyde cross-linking is thought to result from acid induced hydrolysis of the wood carbohydrates, particularly hemicelluloses, which are of structural importance in the wood. The accompanying loss in desirable wood properties has severely limited the commercial acceptability of conventional aldehyde cross-linking processes for the stabilization of wood, and given rise to a recognized need for an entirely different method of cross-linking which will form stable reaction products without the use of degradative catalysts.
It has now been determined that the foregoing problems can be overcome and wood can be effectively and economically beneficiated by treating the wood with an agent capable of forming a borate ester linkage between hydroxyl groups of the cell wall constituents of the wood, and thereafter treating the wood with an aldehyde to effect aldehyde cross-linking of cell wall structural constituents of the wood without obtaining a substantial degradation of desirable wood properties, such as toughness and abrasion resistance. Cross-linking is obtained in the absence of an acid catalyst in the sense of conventional acid catalyzed aldehyde cross-linkage reactions. Rather, it appears that the agent forms an intermediate boric acid ester cross-linkage between free hydroxyl groups in the wood cell wall structural constituents, which is subsequently displaced by an alkyl cross-linkage upon subsequent treatment of the wood with an aldehyde.